This invention relates to supports for strengthening rock structures, and, more particularly, to an improved chemical or resin type anchoring bolt assembly.
The roofs and walls of coal mines, subway tunnels, and other subterranean structures often require anchor bolts to strengthen the rock mass. Two basic anchor bolt designs are currently in use; mechanically operated anchor bolts and chemical or resin anchor bolts. Mechanical anchor bolts typically include a threaded shaft having a leading end which is disposed within a split sleeve or cylinder and inserted into a bore formed in the rock structure. When the shaft is rotated, the sleeve is urged apart and engages the sides of the bore in the rock structure to maintain the shaft in place. The outwardly extending end of the shaft includes a washer or bearing plate which contacts the outside surface of the rock structure as the shaft is rotated to provide the necessary tension pullup or tightening down of the shaft. Standards promulgated by the Bureau of Mines require installed bolt tension to be on the order of 50% of the yield strength of the bolt.
The primary disadvantage of mechanical-type anchor bolts is that they require relatively strong and stable rock stratum to provide the tension on the order of 50% of yield strength of one bolt. In many applications, looser type of formations are encountered such as mudstone or siltstone, limestone, sandstone, or shale. Weaker rock formations of this type are not adequately supported by mechanical anchor bolts.
As a result of the development in recent years of fast setting resin compounds and the inadequacy of mechanical anchor bolts in weaker stratum, chemical type anchor bolts have been developed in which the bolt is secured within a bore drilled into a rock formation by a resin material. Most chemical type anchor bolts employ a cartridge or capsule of polyester resincatalyst material which is first inserted into the bore in the rock formation. The leading end of the anchor bolt is advanced into the bore to pierce the resin-catalyst cartridge and then it is rotated to mix and disperse the material or anchoring cement within the bore and along the bolt. After a predetermined period of mixing, the bolt is held in place to allow the resin to set. The trailing end of the chemical anchor bolt extends outwardly from the bore when the bolt is properly positioned, and, in some designs, is threaded to receive a nut having a faceplate washer. After the cement has completely set to fix the anchor bolt in place, the nut is then independently rotated and advanced along the trailing end of the bolt until the faceplate engages the rock mass and provides tension pullup or tightening down of the bolt.
Proper installation of this type of chemical-type anchoring bolt thus involves a two stage rotational movement of the bolt and nut. Initially, the bolt and nut are rotated together as a unit so that the resin and catalyst in the rock bore are properly mixed and distributed therealong. Once the cement has set and fixed the bolt in place, additional torque is applied to the nut to advance it along at least a portion of the trailing end of the bolt.
Several prior art designs have been proposed for a chemical anchor bolt structure which permits a two stage rotational movement such as described above. Most of these designs are directed to providing means for releasably securing the nut or movable element of the anchor bolt to the trailing end of the bolt shaft, or fixed element, so as to permit unitary movement of the shaft and nut as the resin-catalyst are being mixed, and thereafter allowing the nut to release from the bolt without breaking the bond between the resin and leading end of the bolt. In U.S. Pat. No. 3,877,235 to Hill, for example, the leading end of a cylindrical anchor having a welded end piece at its trailing end is inserted within a bore formed in a rock structure. The threaded shank of a bolt is threaded through a nut mounted to the trailing end of the cylindrical anchor and the head of the bolt extends outwardly from the rock bore and attaches to a bearing plate or washer. The bolt and anchor move as a unit while the resin-catalyst within the bore is mixed to form the anchoring cement. After the cement sets to secure the anchor in place within the bore, the bolt is rotated independently of the anchor so that its inward end pierces the welded end piece of the anchor permitting the washer at the head of the bolt to be tightened against the rock face. In this design, the welded end piece of the anchor permits rotation of the bolt independently of the anchor as the anchoring cement is mixed, but is adapted to break away and allow the bolt to advance within the anchor after the cement has set.
Another common design of chemical anchoring bolts is disclosed, for example, in U.S. Pat. Nos. 4,023,373; 4,122,681 and 4,132,080 which describe a modification of the Hill anchor bolt. As in Hill, a threaded bolt is provided which extends into the bore of the rock structure through a nut mounted at the trailing end of an anchor disposed within the bore. The head of the bolt extends outwardly from the bore and attaches to a bearing plate. Unlike Hill, the anchor of such patented designs does not include a welded end piece; instead, the bolt threads or the threads of the nut attached to the anchor are deformed in some manner to prevent advancement of the bolt through the nut unless a predetermined torque is applied. In operation, both the bolt and anchor rotate as a unit while the adhesive is mixed and distributed along the bore, but after the adhesive is permitted to set, additional torque is applied to the bolt to overcome the interference caused by the thread deformation so that the washer may be advanced into contact with the rock structure.
Further examples of chemical anchor bolts are found in U.S. Pat. Nos. 3,702,060; 3,940,941; and 3,979,918. These patents disclose a shaft threaded at each end, and its leading end is inserted within a bore formed in the rock structure. The threaded, trailing end of the shaft extends outwardly of the bore and is adapted to receive a nut. In each patent, an exposed nut at the end of the bolt shank functions for mixing the resin and, after the resin sets, enables the bolt structure to be tightened to place tension on the bolt. The structures disclosed in these patents all have in common a shaft and nut which are rotated in unison to mix a resin-catalyst material placed in the rock bore, and after the resin has set to fix the shaft in place, sufficient directional torque is applied to the nut to overcome the resistance created by a mechanical deformation or stop for the nut so that a face-plate or washer may be advanced by the nut to contact the surface of the rock structure. Another example of a chemical anchor bolt is shown in U.S. Pat. No. 4,303,354.
There are may disadvantages associated with known chemical-type anchoring bolts where the temporary or the threaded anchor bolt and nut permit unitary movement of the two elements to mix the resin and then separate motion of the nut after the resin is set in place within the rock bore to tighten the bolt. All of the means for interfering with the motion of the bolt relative to an anchor, or the nut along the threaded shaft, mentioned above, are mechanical or structural. In practice a mechanical distortion or stop in the thread of the bolt which cooperates with a nut does not provide an effective fail-safe anchor bolt for the miner who works in very cramped conditions and relies upon his sense of touch. Frequently, distortion of threads gives the miner a false torque of when the nut is tightened which makes the miner believe the bolt is tensioned when, in fact, it is not. On the other hand, if the nut turns freely in the bolt shaft due to a defective thread, the miner may believe the resin is being mixed with the curing agent when, in fact, it is not. In other words, currently available bolt and nut devices which have a mechanical or structural connection between the bolt and nut are not fail-safe devices. Such devices are also subject to variations in tolerances or environmental conditions which could affect the torque required to break the movable element free of the fixed element for movement therealong. If a greater torque than desired is required to break the movable element free, the integrity of the bond between the leading end of the anchor or shaft and the cement may be weakened rendering the anchor bolt useless. On the other hand, if too little torque is required to break the structural connection between the bolt and anchor, or nut and shaft, the bolt or nut may become detached while the resin is being mixed and prematurely advance within the anchor, or along the shaft, before the cement has had time to set.